The present invention is related to the use of shape memory materials in medical devices.
A shape memory material can assume an initial high temperature configuration and a deformed low temperature configuration and revert back to the initial high temperature configuration upon the application of heat. The high temperature configuration of the material is set into the material or memorized during an initial shaping step in which the material is maintained at a high temperature in a desired shape for a period of time.
This shape memory exhibited by metals results from the metal undergoing a reversible solid state phase transition. Specifically, the metal transitions from an austenitic state to a martensitic state with a decrease in temperature. The temperature at which the transition begins is typically designated Ms (martensite start temperature) while the temperature at which the transition finishes is designated Mf (martensite finish temperature). As the metal transitions from the austenitic state to the martensitic state it becomes more easily deformed. In the martensitic state, the metal is able to accommodate significant plastic deformation at an almost constant stress level.
Upon heating a metal in the martensitic state, the metal begins to return to an austenitic state. The temperature at which this transition begins to occurs is designated As (Austenitic start temperature). The transition is complete at a temperature designated Af (Austenitic finish temperature). When Af is attained, the metal has fully reverted to its initial, high temperature configuration.
Shape-memory materials have been disclosed in U.S. Pat. No. 3,012,882 to Muldawer et al. and U.S. Pat. No. 3,174,851 to Buehler et al. both of which are incorporated herein by reference in their entirety.
A variety of materials exhibit shape memory properties including binary metals such as Nickel-Titanium alloys including Nitinol. Doped Nickel-Titanium alloys may also exhibit shape-memory properties.
The use of shape-memory metals in medical applications has been disclosed in a number of references including U.S. Pat. No. 5,197,978 to Hess, U.S. Pat. No. 5,540,712 to Kleshinski et al., U.S. Pat. No. 5,597,378 to Jervis, U.S. Pat. No. 5,769,796 to Palermo et al., U.S. Pat. No. 5,846,247 to Unsworth et al. The contents of the above patents are incorporated herein in their entirety by reference.
Shape-memory materials have been used, inter alia, in the production of stents. Nitinol stents which are fully expanded in the austenitic state and compressed or partially expanded in the martensitic state have been disclosed. The specific Nitinol alloy is chosen such that the stent will be in the austenitic state at body temperature. Prior to insertion into the body, the stent is maintained at low temperature corresponding to a temperature within the martensitic range. Upon delivery to a desired bodily location, the stent is warmed to at least the Af temperature and in the process expanded to its desired final diameter.
Shape-memory metals have been disclosed for use in other medical devices as well.
The physical properties, including elasticity and stiffness, of a shape-memory metal may be controlled via a variety of factors including the chemical composition of the alloy and the treatment to which the alloy is subjected. Commonly assigned U.S. patent application Ser. No. 09/008,684, incorporated herein in its entirety by reference, discloses a process for improving the ductility of Nitinol via control of a number of process parameters. The physical properties of a shape-memory material may also be controlled by heat treating the material. By selective local heat treatment of a shape-memory material, it is possible to destroy the austenitic-martensitic transition and/or change the elasticity and stiffness.